In a method for positively feeding an elastic yarn to the knitting system of a circular knitting machine as known from PCT/EP86/00305, the start and stop signals for the control unit are derived from the movements of the yarn guide for the lead-in and take-out, respectively, of the elastic yarn. On appearance of the lead-in signal, the control unit starts the drive motor of the yarn feeder apparatus at a predetermined speed. The speed is modulated within a predetermined range in response to the successively generated position signals of the circular knitting machine to thereby achieve an accurately synchronized positive yarn feed to the knitting system, taking into account the operating stroke of the needles. Independent of the speed control of the drive motor, the appearance of the take-out signal causes the control unit to generate a stop signal for stopping the drive motor, so that the positive yarn feed terminates with the take-out of the yarn. During the knitting operation, the yarn between the yarn feeding apparatus and the knitting system may be under considerable tension, because the tension of the yarn is still further increased from the take-out instant until the yarn is cut off, due to the drive motor having been stopped, it is extremely difficult to retain the cut-off yarn in the customary yarn clamps (mechanical and/or vacuum-operated). This is due to the tension of the yarn and its compressibility which cause the yarn to slip from the yarn clamp, so that it is no longer available for the next lead-in operation.
Known from DE-OS 19 00 722, is a method for positively feeding an elastic yarn to the knitting system of a circular knitting machine. The yarn is temporarily fed at an increased speed after it has been taken out and before it is cut off, to thereby substantially release the tension of the yarn, so that it is safely retained by the yarn clamp. In this case, however, the operating principle of the purely mechanical yarn feeding apparatus is mechanically derived from the operation of the circular knitting machine, resulting in several disadvantages. The yarn feeder apparatus is only stopped by releasing a clutch after the yarn has been cut off. While the accelerated yarn feeding operation has been initiated at the take-out instant, the accelerated yarn feeding operation is not dictated by the characteristics of the yarn, i.e. its elasticity and smoothness, and the length of the yarn between the yarn feeder apparatus and the knitting system. Feeding is dictated exclusively by the interval between the take-out operation and an instant after the yarn has been cut off, and by the operating speed of the circular knitting machine. The actual tension of the yarn to be relaxed has thus no immediate influence. Although this known method at least partially solves the problems concerning the clamping of the elastic yarn, it is practically impossible to adjust the surplus yarn feed relative to the characteristics of the yarn and to its length between the yarn feeder apparatus and the knitting system. This method is also immediately dependent on a mechanical drive transmission between the knitting machine and the yarn feeder apparatus, which is no longer practicable in the case of a circular knitting machine with an electronically controlled yarn feeder apparatus. The mechanical principle employed in this method, namely, to stop the yarn feeder apparatus only after the yarn has been cut off, and to continue the drive transmission up to this instant, even at an increased yarn feeding speed, is totally unsuitable. This is particularly true when knitting machines are refitted or equipped with electronically controlled yarn feeder devices. The control unit has to perform control functions in response to the take-out signal, and is thus dependant on the timely apprearance of the take-out signal. It is an object of the present invention to provide a method of the type defined in the introduction, which is capable in a simple manner and without any fundamental modifications of the positive yarn feeding process under electronic control of ensuring that the elastic yarn is reliably clamped after having been taken out, independent of its characteristics and of its length between the yarn feeder apparatus and the knitting system. It is also an object to provide a circular knitting machine capable of reliably avoiding the occurrence of knitting faults due to escaping yarn ends, particularly when equipped with an electronically controlled yarn feeder apparatus. In all of its embodiments, the invention is intended to comply with the basic requirement that, after having been taken out, the elastic yarn shall not unacceptably sag on being led in again, and that between the take-out and lead-in operations the yarn is relaxed to the degree necessary for preventing it from being pulled out of the yarn clamp.
These objects are attained according to the invention by the characterizing features discussed below.
The control unit is capable of performing the usual control functions on appearance of the take-out signal. The drive motor of the yarn feeder apparatus is not yet stopped at this instant, however, because the control unit is prevented from generating the stop signal for the drive motor in response to the take-out signal until a predetermined number of position signals of the circular knitting machine has been received, or until a predetermined interval has expired. During this phase of the method the yarn continues to be positively fed at a rate dictated by the characteristics of the yarn and/or by the length of yarn between the yarn feeder apparatus and the knitting system. The control unit operates to stop the drive motor only after the predetermined number of position signals has been received or after the predetermined interval has expired, respectively. In this manner it is ensured that the yarn has been sufficiently relaxed, depending on its quality and the distance between the yarn feeder apparatus and the knitting system, so that it can be properly clamped after having been cut off. The surplus yarn feeding operation may in this case be continued beyond the instant at which the yarn is cut off and clamped to thereby reliably release its tension. When making use of the position signals, the surplus yarn feeding operation is independent of the operating speed of the circular knitting machine, because the predetermined number of position signals of the knitting machine is solely dependent on the displacement of the knitting cylinder relative to the knitting system. Sufficient relaxation of the yarn may be likewise achieved by properly selecting the interval to expire prior to the generation of the stop signal. The method is suitably applicable to any circular knitting machine equipped or refitted with electronically controlled yarn feeder devices.
Since the control of the drive motor of the yarn feeder apparatus is mechanically independent of the operation of the circular knitting machine, the method permits the surplus yarn length fed to be accurately adjusted to the yarn characteristics and/or to the respective knitting machine, with particular consideration as to whether the yarn is an extremely elastic, naked cord (rubber or lycra) or a less elastic textile-covered thread. The construction of the positive yarn feeder apparatus is not of importance i.e. the method is successfully applicable both to the positive yarn feeder devices in which a yarn supply is unwound by a roller resting on the yarn supply or by means of squeeze rollers, and to yarn feeder devices in which a yarn supply consisting of only a few yarn windings is formed on a drum rotated by the drive motor. The operation of the circular knitting machine does not have to be modified, so that the method is particularly suitable for refitted circular knitting machines or such machines intended to be refitted.
In a suitable embodiment of the method according to the invention, the predetermined interval is represented by signals of a timing signal sequence dependant on the characteristics of the elastic yarn and/or the distance between the yarn feeder apparatus and the knitting system. It is similarly also possible, however, to generate a single signal denoting the end of the interval after the take-out signal for causing the stop signal to be generated.
According to another important variation of the method, the operating speed of the drive motor may be increased or decreased relative to its operating speed prior to the appearance of the take-out signal, with the motor operating at this increased or decreased speed until generation of the stop signal. The electronic control unit is particularly well suited for this purpose, particularly when it is equipped with a programmable memory or microprocessor permitting it to control the drive motor independent of the operation of the circular knitting machine during this phase. The surplus yarn feeding operation is then carried out at a higher speed than normal for a rapid yarn relaxation or at a lower speed than normal for delayed yarn relaxation. The accelerated surplus feeding operation may already be terminated before the yarn is cut off. In the case of delayed surplus feeding operation, the relaxation period continues after the yarn has been cut off.
A circular knitting machine according to the present invention offers the advantage of highly reliable operation when processing elastic yarns, independent of the characteristics of the yarn being processed and of the distance between the yarn feeder apparatus and the knitting system. The delay circuit in the connection to the drive motor ensures that the drive motor continues to operate for the surplus yarn feeding operation after the take-out signal has been generated, until the yarn extending to the knitting system has been relaxed and can therefore be safely clamped. Most of the components employed are required for the operation of the yarn feeder apparatus in any case. The adaptation of the yarn feeder apparatus to the circular knitting machine (signal generators for position, take-out and lead-in signals) is very simple, so that already existing circular knitting machines may be readily re-equipped. The delay circuit can be integrated in the drive and control system of the yarn feeder apparatus without affecting the conventional control functions.
The delay circuit is suitably provided with a driver circuit for the take-out signal or stop signal, respectively, which is timed by the position signals of the circular knitting machine. The driver circuit delays the stop signal for the drive motor by a predetermined number of position signals to thereby ensure that the yarn extending to the knitting system is properly relaxed and can thus be safely clamped when the drive system of the yarn feeder apparatus is finally stopped.
For the universal adaptation of the circular knitting machine to any operating conditions it is advantageous to provide the delay circuit with selector means for the number of position signals to be counted or for the length of the interval after generation of the take-out signal. The selector means permits the operation to be adjusted to the characteristics of the yarn employed and/or to the distance between the feeder apparatus and the knitting system and/or to the yarn tension required for normal operation and intended to be relaxed, without affecting the circular knitting machine in any manner.
It may finally be advantageous to adjust the operating speed of the drive motor to a value deviating from the operating speed prior to the appearance of the take-out signal on activation of the delay circuit. With the provision of this adjustment it is possible to pre-select the speed of the yarn surplus feeding operation for relaxing the length of yarn between the yarn feeder apparatus and the knitting system. The yarn may thus already be relaxed before being received by the yarn clamp, or may alternatively be fully relaxed only after having been clamped under a certain tension, which may be helpful for the clamping operation, so as to be subsequently prevented from being pulled out of the clamp.